The present invention relates to a process for enhancing the electroplating of non-conductive surfaces, such as the through holes of a printed circuit board (PCB). The inventive process includes depositing on the non-conductive surface a layer of carbon (e.g. graphite, carbon black or some other intermediate carbon structure) which carbon has been specially modified. It has been found that the practice of this invention eliminates the need for electroless plating of the non-conductive surface, while reducing the covering time, and enhancing the coverage and ease of plating as compared to the use of unmodified carbon as the initiating layer.
Printed circuit boards are generally composed of a non-conductive layer, such as an epoxy resin/glass fiber mixture, which is positioned between copper or nickel plates or foils, or other conductive metal layers. There can also be a multiplicity of these alternating layers. Commonly, holes are drilled through the PCB to establish a connection between the conductive metal layers at specific points in the board. The holes are then metallized to form a connection between the conductive materials, usually by plating.
In order to achieve conductivity and consistent and reliable bond between the electroplated metal (usually copper), and the through holes, the through holes are usually first provided with a layer of electroless copper in a process which requires several steps, including pre-activation, activation with a palladium/tin activator, application of an accelerator, electroless copper deposition and one or more rinses, before electroplating could be effected.
The need for application of electroless copper can be avoided, it has been found, by the deposition of carbon black on the through holes or other non-conductive surface which is to be electroplated. In this way, the long process time, complex chemistry requiring constant monitoring, and sensitivity of electroless baths to contamination can be avoided. Moreover, the expensive waste treatment often required with electroless copper and palladium/tin activators can be eliminated.
However, the use of a carbon black deposition process has several drawbacks. After carbon black deposition, it takes several minutes before the non-conductive surface being plated is completely covered by the electroplated metal. This is especially significant where the surface to be plated is a through hole. Electroplating after treatment with carbon black begins adjacent to the outer conductive surfaces (i.e., the copper foil) of the PCB and extends inward towards the center of the hole. This occurs from both sides of the through hole and the plating meets in the center and completes the connection. In addition, it is sometimes difficult to achieve complete coverage and acceptable plating particularly in small, high aspect ratio holes. Finally, possibly, because of the resistance of the carbon black layers, it can be difficult or impossible to plate large areas of non-conductive surfaces, as are required in general plating on plastics.
This "bridging" of the hole is a slow process and provides unwanted opportunities for defects in the metal plate, such as voids. In addition, because of the time it takes for "bridging" to occur after carbon black deposition, carrying out the process in a conveyorized manner can be commercially impractical, and hampers the ability to add a conveyorized electrolytic copper flash immediately after carbon black treatment.
Prior to the development of electroless methods of plating through holes, it was suggested to use graphite to prepare the walls of the through holes prior to plating. Radovsky et al. in U.S. Pat. No. 3,099,608, teaches a process for preparing the through hole walls of a printed circuit board by initially depositing a thin electrically non-conductive film of palladium metal in at least a semi-colloidal form in the through holes. It is indicated by Radovsky et at. that graphite had been used previously as a conductive base layer for electroplating (see col. 1, lines 63-70, and col. 4, line 72--col. 5, line 11 ). It is noted by Radovsky et at. that there are several defects with the prior art graphite process including lack of control of the graphite application, poor deposit of the resultant electroplated metal, non-uniform through hole diameters, and low electrical resistance of the graphite. It is also mentioned by Shortt et at. U.S. Pat. No. 3,163,588 that graphite (or equivalents of graphite) may be employed to render through hole walls of electric circuit boards conductive for later electroplating metals thereon.
Graphite has also been employed in numerous other types of processes for preparing a non-conducting material for a metal coating or plating. For instance, U.S. Pat. No. 409,096 to Alois Blank teaches a process for applying copper to asbestos roofing material. The process involves applying a powdered plumbago (graphite) in a volatile liquid such as varnish to the surface of the asbestos, then evaporating the volatile liquid to coat the asbestos fibers with particles of plumbago. The graphite coated asbestos sheets are then immersed in a copper solution and electric current is applied to form a thin film of copper on the sheets. The copper coated sheet is then immersed in a bath of molten metal such as tin, lead, or zinc, and is removed from the molten bath to effect solidification. The resulting metal coated asbestos sheet is described as being relatively flexible, a non-conductor of heat and substantially fireproof.
In U.S. Pat. Nos. 1,037,469 to Goldberg and 1,352,331 to Unno, processes for electroplating non-conducting materials are described, which involve coating the material with wax and then a slurry of particles of graphite or other metal, followed by electroplating of the coated surface with copper or other metal. Neither of these processes are particularly suitable for use in coating the hole walls of circuit boards because the holes are normally extremely narrow in diameter and immersing in wax would tend to plug the hole and prevent coating the through hole walls with an electroplating material.
A process which involves "graphiting" a thin layer onto the non-conducting surface followed by applying a copper layer electrolytically and "finally a further electrolytic deposit of another metal" is disclosed by Laux in U.S. Pat. No. 2,243,429.
The first practical teaching of a carbon black deposition system, which permits the elimination of electroless copper deposition prior to electroplating, was by Minten and Pismennaya in U.S. Pat. No. 4,724,005, the disclosure of which is incorporated herein by reference in its entirety. Although the carbon black process they disclose is effective, the long "bridging" time (ie. the time necessary to fully connect both sides of the hole with the plated metal or to achieve substantial coverage) created is not recognized, nor any solution proposed. A long line of patents have issued concerning improvements to or variations in the process described in U.S. Pat. No. 4,724,005 including U.S. Pat. No. 5,139,642 to Randolph et al., the teachings of which are incorporated herein by reference in their entirety, but none of them address or relate to the invention at hand.
What is desired is a carbon treatment process which is effective for promoting the electroplating of a non-conductive surface, while reducing the "bridging" time observed when electroplating.